1. Field of the Invention
The present invention generally relates to fastener and washer assemblies used as securing or fastening devices. More specifically, this invention relates to a fastener and washer assembly in which the washer is permanently retained on the shank of the bolt by a raised periphery, wherein the raised periphery is formed on the shank of the fastener prior to installing the washer and wherein the washer is provided with a lead chamfer on its internal diameter which enables the washer to be elastically forced over the raised periphery of the fastener so as to be permanently retained thereby.
2. Description of the Prior Art
Pre-assembled fastener and washer assemblies have gained wide acceptance in manufacturing because of the convenience and efficiency they offer, providing the manufacturer with savings in both assembly time and costs. The fastener can be either a screw or a bolt which is intended to receive a nut. Fastener and washer assemblies are commonly available in many sizes to accommodate applications ranging from electronics to automobile assemblies. Such applications generally demand a clamping capability which can be maintained in an environment which includes vibration and temperature variations that tend to fatigue or otherwise decrease the fastener's ability to sustain the desired clamping load. Accordingly, the fastener and washer must each possess specific material characteristics suitable for the particular application and its environment. However, the material characteristics of the fastener and washer, such as strength, stiffness, and heat and surface treatments, are ultimately dependent upon the method used to manufacture the assembly. Consequently, the processing techniques used to form the fastener and washer and assemble them together can be critical.
Where material characteristics, particularly surface and heat treatments, of the fastener and washer are not critical, the washer can be assembled to the fastener prior to the final processing steps of the fastener. Such an approach is taught in U.S. Pat. Nos. 2,303,224 to Olson; 2,792,578 to Autio; 2,914,780 to Poupitch; 3,862,458 to Stanaitis; and 3,837,212 to Gutshall. Olson teaches assembling the washer to the fastener, and thereafter retaining the washer on the fastener by forming a raised knurled rib on the perimeter of the fastener's shank to trap the washer between the fastener head and the knurled rib. Similarly, Autio teaches forming intermittent protuberances on the perimeter of the fastener's shank after the washer has been assembled to the fastener, while both Stanaitis and Gutshall form a flange on the fastener's shank for retaining a previously assembled washer.
In contrast, Poupitch teaches actually forming the fastener after the washer is in place such that the washer is retained against the fastener head by a protrusion on the shank's perimeter. Further variations of this approach are illustrated by U.S. Pat. Nos. 1,262,975 to Pierce and 1,868,415 to Gundersen et al. Pierce actually welds the shank of the fastener to the fastener head in a manner which rigidly secures a washer to the fastener. Annular reliefs formed along the inner diameter of the washer receive molten metal from the shank and fastener head during welding to positively retain the washer. Gundersen et al initially forms the washer as an integral part of the fastener, and thereafter shears the washer from the shank of the fastener in a manner that produces a tapered inner diameter to the washer. The threads of the fastener are formed so as to be greater in diameter than the inner diameter of the washer to positively retain the washer to the fastener.
A disadvantage to all of the above approaches is that the material characteristics of the fastener and washer are severely limited in that the final processing steps occur after the washer is secured to the fastener. In particular, any heat treating or surface treatments desired after the final forming steps must be carried out on both the fastener and washer, whether to do so is desirable or not. The alternative is to forego any subsequent heat treating or surface treatment. However, this is highly undesirable under many circumstances, particularly where improved material characteristics would allow the use of smaller fasteners which provide for substantial weight and space savings. In addition, surface and heat treatments are desirable where the environment is particularly corrosive or is subject to temperature extremes.
Accordingly, forming the fastener and washer during entirely different steps and from different materials allows greater versatility in the material qualities and uses of the fastener and washer assembly. For instance, the fastener can be formed to have optimal strength while forming the washer from a more resilient material. Further, the fastener and washer can separately undergo heat and surface treatments particularly suited for the materials used and the intended environment.
One solution for achieving this advantage is to retain the washer on the fastener with a third member, such as a collar. This type of approach is illustrated in U.S. Pat. No. 3,687,184 to Wagner. Wagner teaches the use of a retainer member having an annular shoulder which is slightly greater than the inner diameter of the washer. The washer is first assembled to the retainer member, being forced over the annular shoulder so as to retain the washer directly to the retainer member. The retainer member is then assembled to the fastener, being retained on the fastener with a slight interference between the inner diameter of the retainer member and the fastener threads. However, a clear disadvantage with the teachings of Wagner is the need for an additional member, incurring additional part costs and assembly time. Another possible disadvantage is the potential adverse effects on the clamping capability of the fastener and washer assembly.
As an alternative, U.S. Pat. No. 2,283,494 to Erdman teaches the preforming of an annular shoulder on the shank of a fastener prior to assembling the washer. The washer is described as being forced over the annular shoulder so as to be permanently retained thereby. In practice, this approach requires that the tolerances between the diameter of the washer and the shoulder must be closely held to allow the washer to be forced over the shoulder without significantly deforming the washer. Such tight tolerances are undesirable, as they are expensive and difficult to maintain in a mass-produced item such as a fastener and washer. It is a likely tendency that the pressing procedure necessary to force the washer onto the fastener requires a substantial force, resulting in the inner diameter of the washer being deformed. Such a result is even more likely where the fastener or the washer has been hardened through a heat treatment or surface treatment process. A supplemental approach to diminishing the likelihood of permanent deformation to the washer is to limit the washer material to a spring steel, and further to limit the thickness of the washer to enhance its resiliency. However, any attempt to facilitate assembly by altering the material characteristics severely limits the choice in materials, limiting the types of applications for which the fastener and washer assembly are suitable.
From the above discussion, it can be readily appreciated that the prior art does not disclose a fastener and washer assembly which possesses the advantages of allowing the fastener and washer to be independently processed prior to assembling, while also facilitating assembly such that additional members and undersirably tight tolerances are avoided. Nor does the prior art teach or suggest such a fastener and washer assembly which avoids the requirement of excessive assembly forces which have the tendency to damage and otherwise deform the members of the assembly.
Accordingly, what is needed is a method of producing a fastener and washer assembly which allows the fastener and washer to be formed and processed independent of each other, yet provides that the fastener and washer are formed such that they can be readily assembled together inexpensively even after such processing as heat and surface treatments. As an advantageous result, the fastener and washer can be manufactured and assembled with conventional tolerances, which facilitate mass production.